The present invention relates to metal casting, and more particularly, to a method for casting a fine-grain ingot.
The production of ingots by continuous casting is well known in the prior art. Generally, a continuous casting process employs a mold having a cooled outer wall and a movable bottom, or plug. Molten metal is poured into the top of the mold in a vacuum enclosure. As the metal solidifies, it is drawn downwardly by the plug while at the same time, additional molten metal is poured into the mold at the top.
Because heat loss from the ingot in this type of continuous casting process occurs primarily at the cooled mold walls and, downwardly, through the solidified portion of the ingot, the solidification of the molten metal in the newly poured ingot occurs at relatively low rates; for example, movement of the liquid-solid interface at rates slower than approximately 1/10 inch per minute in the central regions of the ingot is typical. For many materials, and particularly more complex alloys, the relatively slow solidification rate is accompanied by the growth of dendritic crystals having large arm spacings, and by significant segregation of various alloy constituents in the regions between the dendritic arms.
Conventionally cast ingots having dendritic crystal and segregation imperfections of the type mentioned above usually require heating--for example, at temperatures slightly below the alloy's solidus temperature, for periods of up to 24 to 36 hours--prior to being subjected to mechanical hot working operations such as rolling and forging. Even then, hot working of conventionally cast ingots of many complex alloys may be accompanied by so much surface cracking that some of these ingots are considered to be unworkable.
Another problem associated with conventional continuous casting processes known in the prior art is the formation of ruptures in an ingot sidewall during ingot casting. The ruptures, or so-called hot tears, are formed by frictional forces between the ingot and mold when the ingot is lowered in the mold before its sidewall regions have cooled sufficiently. For most purposes, hot tears constitute an unacceptable sidewall condition where further processing of the ingot is required.
A number of casting techniques for producing ingots which have reduced hot-tear and segregation problems have been proposed. Some newer casting techniques are designed particularly for the production of high quality, ultrahigh-strength alloy ingots which are suitable for rolling, forging or the like. U.S. Pat. No. 3,709,284, discloses a continuous casting method in which a water-cooled ram or plug periodically engages the top of the ingot during casting, to cool the ingot from its upper surface. The method involves contacting the cooling plug with each newly poured molten-metal layer, which may have a thickness of about 1/16 inch. Electron beam heating is used to heat the ingot's upper surface between solidification operations, to assure good bonding between the successive layers.
As the plug makes repeated contact with the upper surface of the newly poured increments, it begins to collect a surface contamination coating or deposit which is formed, in part, from metal vapors from the molten alloy. Since the coating which collects on the plug has a different composition than that of the alloy itself, the plug must be cleaned periodically to prevent the material from being introduced into the ingot melt. The need to keep the plug surface clean adds to the complexity and expense of the operation, and unless the plug is kept completely free of vapor coatings, some contamination of the ingot will occur. This process, therefore, is best suited for high-strength steels and other alloys that do not need to be ultra-clean.
In a second method which has been proposed for production of relatively uniform-grain ingots, partially molten material from a pair of consumable electrodes, heated by vacuum arc melting, drips onto the central upper surface of an ingot being formed in a spinning mold. As a partially molten drop hits the ingot surface, at the center of the spinning mold, it spreads out in a thin layer which covers the entire ingot upper surface.
Ingots produced by the spinning mold process may lack fine grain size, typically exceeding ASTM 3-4. The heated material which drops onto the ingot never reaches the liquidus temperature, and therefore the thin layers forming the ingot contain unmelted solid particles which can seed larger grains in the solidified ingot. The need for high rotational speeds in this process also introduces significant mechanical complexity to the apparatus.
Ultrahigh-strength alloys having a fine-grain crystalline structure may be produced by powder metallurgy. The powdered alloy can be converted to the equivalent of a billet by means of conventional hot pressing techniques, and such billets can then be converted to forged parts that exhibit excellent mechanical properties. However, powder metallurgy methods typically provide a relatively low yield of usable powder, and thus material costs are high. Additionally it is difficult to prevent damaging impurities from contaminating the powder.
It is one general object of the present invention to provide an improved method for producing fine-grain, high-strength alloy ingots.
A more specific object of the invention is to provide a method for producing an high-strength iron, nickel or cobalt-based ingot which can be hot rolled or forged directly without the need for extensive prior heat treating the ingot.
A related object of the invention is to provide a method for producing such an ingot that has a crystal grain size between about ASTM 5 and 7.
Yet another object of the invention is to provide a method for producing such an ingot of relatively large diameter, i.e., substantially greater than 6 to 8 inches (15 cm to 20 cm).
Still another object of the invention is to provide a method for producing such an ingot having a hollow interior.
It is still another object of the invention to provide, by such method, a high-strength iron, nickel or cobalt-based ingot having a grain size between about ASTM 5 and 7 when viewed on a surface cut transverse to the longitudinal axis of the ingot, and consisting of longitudinal grains ranging in length from about 1 mm to about 20 mm parallel to the longitudinal axis of the ingot.
According to the method of the invention, a feedstock stick is melted to produce either a continuous stream of molten metal or a series of fully molten drops. The metal falls on the upper surface of an ingot being formed, to cover a portion thereof which is substantially less than the ingot's total upper surface. The mold is moved laterally with respect to the feedstock stick so that the molten metal impinges upon different portions of the ingot's upper surface. The melt rate is so selected that the impact region on the ingot's upper surface is at or below the solidus temperature of the alloy and above a temperature at which metallurgical bonding with the impinging metal can occur.
The apparatus for carrying out the method of the invention includes a support for holding the feedstock stick, an electron beam for heating the stick, and an ingot mold which is shiftable laterally with respect to the support.
These and other objects and features of the present invention will become more fully apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.